Outmigrant Trapping in the Napa River – How to Start a Monitoring Program from Scratch

The Napa County Resource Conservation District (RCD) has operated a rotary screw trap in the mainstem Napa River since 2009.  This program represents the first outmigrant trapping effort ever undertaken for the Napa River basin.  In 2010, a total of 25 fish species were captured (13 native, 12 exotic).  The total catch was 33,550 fish, which was comprised of 32,426 natives and 1,124 exotics.  Larval fish (< 25mm in length) were a large component of the total catch, including approximately 25,644 Sacramento sucker larvae and approximately 969 largemouth bass larvae.  The total catch of non-larval fish was 6,937.  Native species dominated the total non-larval catch as well (n=6,782), accounting for 97.7% of all non-larval specimens.  It appears that although exotic fishes are present, native species still persist in the freshwater segment of Napa River fish community.  In 2010, a total of 314 steelhead (Oncorhynchus mykiss) were collected, including 223 smolts, 88 parr, and 3 adults (>300mm in length).  The median steelhead smolt length was 198mm, compared to 178 mm in 2009.  A total of 1,371 Chinook salmon (O. tshawytscha) parr and smolts were captured, compared to a single Chinook smolt caught in 2009.  It appears steelhead and salmon smolt size and abundance fluctuates significantly from year to year.

A total of 317 salmonid specimens were captured that could not be readily identified based on morphology.  Initial identification efforts by experts in California and Washington state suggested that the fish were a mix of pink (O. gorbuscha) and/or chum salmon (O. keta).   However, three specimens were determined to be sockeye/kokanee (O. nerka) through genetic marker analysis by the National Oceanic and Atmospheric Administration Fisheries Service (NOAA Fisheries).  Additional genetic analysis is needed to determine whether all of the unidentified specimens were in fact sockeye/kokanee salmon.  A total of 198 steelhead smolts and 702 Chinook smolts were fin-clipped and released approximately one km upstream of the trap to estimate trap efficiency.  Efficiencies were calculated to at 11.1% for steelhead and 19.8% for Chinook.  The Napa RCD and its partners plan to continue operating the trap annually to develop salmonid population indices and track ecological responses to ongoing habitat restoration.  Annual reports are available at www.naparcd.org.